In the recent drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The rapid advance toward finer pattern rules is grounded on the development of a light source of a shorter wavelength, a projection lens with an increased numerical aperture (NA), and a resist material with improved performance.
With respect to the light source for exposure, the change-over from i-line (365 nm) to shorter wavelength KrF laser (248 nm) enabled mass-scale production of DRAM with an integration degree of 64 MB (processing feature size ≦0.25 μm). To establish the micropatterning technology (processing feature size ≦0.2 μm) necessary for the fabrication of DRAM with an integration degree of 256 MB and 1 GB or more, the lithography using ArF excimer laser (193 nm) is under active investigation. Although F2 laser (157 nm) is also considered as one candidate light source of shorter wavelength, the use of F2 laser is postponed because of many outstanding problems including a more expensive scanner.
With respect to the increase of NA, not only an improvement in lens performance is sought for, but also the immersion lithography which can establish an NA of 1.00 or greater by filling a high refractive index liquid between a lens and a wafer is of great interest. See Proc. SPIE, Vol. 5376, p44 (2004), for example. For the ArF immersion lithography now under investigation, it was proposed to apply to the 45-nm node by filling the space between the lens and the wafer with deionized water having a refractive index of 1.44. See Proc. SPIE, Vol. 5040, p724 (2003), for example.
With respect to the resist material, since the development of acid-catalyzed chemical amplification positive working resist materials as disclosed in U.S. Pat. Nos. 4,491,628 and 5,310,619 (JP-B 2-27660 and JP-A 63-27829), it has become possible to achieve a higher resolution and sensitivity. They now become predominant resist materials adapted for deep UV lithography. Of these, the KrF resist materials enjoyed early use on the 0.3 micron process, passed through the 0.25 micron rule, and currently entered the mass production phase on the 0.18 micron rule. Engineers have started investigation on the 0.15 micron rule. The ArF resist is expected to enable miniaturization of the design rule to 0.13 μm or less.
Various alkali-soluble resins are used as the base resin in such chemically amplified resist compositions. Depending on a light source selected for light exposure, a base resin of different skeleton is used. For KrF resists, a polyhydroxystyrene resin having phenolic hydroxyl groups as the alkali-soluble group is now a standard base resin.
For ArF resist materials, since polyhydroxystyrene resins and novolac resins have very strong absorption at a wavelength around 193 nm, studies were made on poly(meth)acrylate resins and resins comprising cycloaliphatic olefin such as norbornene as polymerized units, both using carboxyl groups as the alkali-soluble group (see JP-A 9-73173, JP-A 10-10739, JP-A 9-230595 and WO 97/33198). Of these, the poly(meth)acrylate resins are expected to reach a practical level because of ease of polymerization. One of the poly(meth)acrylate resins proposed thus far is a poly(meth)acrylate resin having methyladamantyl groups as the acid labile group and lactone rings as the adhesive group as disclosed in JP-A 9-90637. Norbornyl lactone is also proposed as an adhesive group having enhanced etching resistance as disclosed in JP-A 2000-26446 and JP-A 2000-159758.
The serious problems left unsolved for ArF resist materials are reduction of line edge roughness and enhancement of etching resistance. In general, a higher light contrast leads to a less line edge roughness. For example, increased NA of lens, application of modified illumination or phase shift mask, or wavelength reduction allows the light contrast to be increased, resulting in a reduced line edge roughness. Thus the wavelength reduction from KrF to ArF excimer laser is expected to reduce line edge roughness. However, it is reported that ArF resists actually have greater line edge roughness than KrF resists and that image contrast is in inverse proportion to line edge roughness. This is attributable to the difference in performance between ArF and KrF resists. See Proc. SPIE, Vol. 3999, p264 (2000), for example.
The use of an alternating copolymer as the base is proposed as one means of minimizing the edge roughness of pattern after development. See Proc. SPIE, Vol. 5039, p672 (2003), for example. The alternating copolymer having an ordered arrangement of recurring units within the polymer chain is characterized by its ability to minimize edge roughness, as compared with random copolymers and block copolymers.
One candidate of alternating copolymers that can be used as the ArF resist is a copolymer of norbornene and maleic anhydride as described in JP-A 10-10739. However, the resist using this copolymer suffers from storage instability and the like, and it remains unexpectable when this resist will be reduced to commercial practice. Another candidate is a copolymer of norbornene and α-trifluoromethylacrylate, which was the base polymer candidate for F2 resist. See Proc. SPIE, Vol. 4345, p273 (2001), for example.